1. Field of the Invention
The present invention relates to gas-monitoring assemblies and methods for selectively determining presence of one or more target gases in a gaseous environment, while reducing cross-interference caused by presence of one or more interfering gases in such gaseous environment.
2. Description of the Related Art
In order to protect workers against potential exposure to toxic and hazardous gases, toxic/hazardous gas monitoring (TGM) devices are commonly installed in workplace and other premises for monitoring the concentration of such toxic and hazardous gases thereat.
Conventional TGM devices employ molecular emission spectrometers for hydrogen-based gas detection. However, the introduction of hydrogen gas gives rise to additional safety concerns and requires implementation of additional safety measures to reduce risks associated with hydrogen gas.
New TGM systems employing electrochemical gas sensors can be advantageously used for monitoring toxic and hazardous gases, in place of the conventional hydrogen-based TGM devices. The use of electrochemical gas sensors eliminates the risks associated with hydrogen gas and therefore results in increased customer acceptance.
In electrochemical gas-sensing systems, a target gas contacts a measuring electrode of the electrochemical sensor, which responsively generates an electrical current that is proportional to the concentration of such target gas. For further details regarding electrochemical gas sensors, see U.S. Pat. No. 6,248,224 issued on Jun. 19, 2001 for “TOXIC SENSOR AND METHOD OF MANUFACTURE” and U.S. Pat. No. 6,423,209 issued on Jul. 23, 2002 for “ACID GAS MEASURING SENSORS AND METHOD OF USING SAME.”
However, gases that belong to the same chemical family often show similar or comparable reactions in the same electrochemical cell and cause cross-interference in the electrochemical measurement results. Currently available electrochemical gas sensors not only respond to the presence of a target gas, but also to presence of other gases of the same chemical family.
For example, an AsH3 sensor responds not only responds to AsH3 gas, but also to other hydride gases such as B2H6, SiH4, and PH3. The measuring electrode of such AsH3 sensor reacts with the AsH3 gas as follows:AsH3+3H2O→H3AsO3+6H++6e−,while it also reacts with SiH4, an interfering hydride, as follows:SiH4+3H2O→H2SiO3+8H++8e−.
Therefore, the interfering gases will trigger the electrochemical gas sensor to generate signals indicating the presence of the target gas, even when the target gas is actually absent (i.e., false alarm).
It is therefore one object of the present invention to provide an apparatus and method for reducing such false alarm and increasing the selectivity of electrochemical gas sensors.
It is another object of the present invention to provide an apparatus and method for increasing selectivity of other types of gas sensors whose measurement results are similarly affected by presence of interfering gases and are prone to false alarm.
Other objects and advantages will be more fully apparent from the ensuring disclosure and appended claims.